Apparatus of the above kind is commonly used in some kinds of motor vehicles in which the cabin is quite large and therefore necessitates a flow rate of heating and/or air conditioning air which is larger than in the case of the majority of conventional vehicles. It has in fact become clear that it is preferable to use two fans with additive air flows, rather than a single fan giving the required increased air flow rate. If only a single fan is provided, in order to obtain such a high flow rate it would be necessary to design a special fan which could not be used on the more usual sizes of vehicles, and which would be somewhat cumbersome and also noisy.
By contrast, by using two fans it is possible to use fans of standard design, that is to say those which can be used on a wide variety of vehicles, and which in addition are of small physical size.
However, the use of two fans in a single installation has a disadvantage which arises from the fact that, due to variations in fan characteristics resulting from manufacturing tolerances, the flow rates of air delivered by the two fans tend to be somewhat different from each other. These fans are driven by direct current electric motors, and for equal voltages, two fans may in fact deliver air at very substantially different flow rates. As a result, it is impossible to arrange two fans in such a way that they will produce exactly the same flow rate, so that therefore the air passing through the heat exchanger has a rate of flow that is not the same over the whole of the surface of the heat exchanger.
In practice, the two air streams delivered respectively by the two fans are channeled by means of vanes which are perpendicular to the inlet direction of the air streams, and which are arranged to direct the two streams respectively towards two halves of the heat exchanger, commonly a right hand half and a left hand half. Thus the flow rate of the air passing through one of these halves of the heat exchanger is not the same as that of the air passing through the other.
This drawback may be further aggravated by the fact that the two fans are generally connected to the housing through two respective ducts which are not necessarily of the same configuration, and which therefore may produce different energy losses due to friction and turbulence.